\def\beamer@ftb@str{
  \setbeamerfont{structure}{series=\bfseries}
  \setbeamerfont{block title}{size={}}
}

\DeclareOptionBeamer{onlysmall}{\let\beamer@ftb@str=\relax}
\DeclareOptionBeamer{onlylarge}{\let\beamer@ftb@tstr=\relax}

\ProcessOptionsBeamer

\mode<presentation>

\beamer@ftb@str

\mode
<all>

